Micturition (Urination) Process – Overview and Control

1. Allowing Bladder to Fill:

   • Sympathetic Nervous System (SNS) uses neurotransmitters Epinephrine, E & Norepinephrine, NE) to create a sympathetic tone on both detrusor muscle and internal sphincter, so that:
     • o SNS relaxes the detrusor smooth muscle of the bladder (allows filling) and
     • o SNS contracts the internal smooth muscle sphincter around urethra (preventing urination).
     • o Cerebrum contracts the voluntary external sphincter (skeletal muscle) around urethra (preventing urination).
   • The bladder contains 300-400 mL of urine before the urge to urinate occurs. Adult bladder can hold more than 500mL.
   • Pelvic Floor Muscles: provide support and control over the bladder as they contract during increases in intra-abdominal pressure (e.g., coughing, lifting) to prevent involuntary urination.

2. Urination:

   • The micturition sacral reflex is initiated when:
     • 1) Stretch receptors in the bladder are activated & the signal is transmitted via sensory afferent neurons to the spinal cord, cerebrum & pons. The L1 sensory nerve sends fullness information to the pons (pontine micturition center).
     • 2) Parasympathetic Nervous System (PSNS) nerves are stimulated by the bladder stretching and release Acetylcholine (ACh) which cause bladder detrusor muscle to contract. However, urination is prevented as sphincters remain closed, though pressure in bladder builds.
     • 3) Cerebrum determines if location is appropriate, then stimulates pons to inhibit SNS output to detrusor & internal sphincter, so that detrusor no longer relaxes and internal sphincter no longer contracts. PSNS is allowed to continue contracting the detrusor.
     • 4) Voluntary motor neuron to external sphincter stops firing so external sphincter relaxes. Urination occurs.